Rose of Sharons and Crepe Myrtles

Hey. I've got a huge Rose of Sharon bush, and I keep seeing more little ones crop up here and there around my yard. Was wondering how can it do that? Pollination from bees? Same with this Crepe Myrtle I've gotten somehow. My neighbor has tons of Crepe Myrtles. And somehow one has found its way to my yard. And again with the Rose of Sharon, my neighbor all of a sudden has one, too.
Don't get me wrong, I love my Rose of Sharos, and my "volunteer" Crepe Myrtle. I know about pollination and such, but didn't think it could regrow one several feet away.

Rose-of-Sharon will send out runners and propagate by seed. My dad had and ancient one that became a real pain because it was all over the place. I had / have one that never seemed to propagate much but it is only 7 years old. Around here there is a virus or fungus that has been hitting these hibiscuses and killed a lot of the old wood on mine. The trunks are rotting 4 to 8 inches above the soil line.